¹²⁵I Radioimmunoassay for the Dual Detection of Amphetamine and Methamphetamine

A radioimmunoassay that exhibits a nearly equivalent response to D-amphetamine and D-methamphetamine in urine over the assay range of 0 to 1000 ng/mL while displaying low cross-reactivity to L-amphetamine and L-methamphetamine (4.6% and 2.4%, respectively) has been developed. In addition, methylenedioxy-amphetamine (MDA) and methylenedioxymethamphetamine (MDMA) were detectable in the assay with cross-reactivity levels of >100% and 77% respectively. Little cross-reactivity was observed with the commonly encountered over-the-counter (OTC) drugs and this cross-reactivity was further reduced by the addition of sodium periodate into the reaction mixture to oxidize the β-hydroxylamines. The double (second) antibody assay uses ¹²⁵I-radiolabeled derivatives of both D-amphetamine and D-methamphetamine as tracers in combination with two highly specific sheep antisera directed against D-amphetamine and D-methamphetamine. The assay exhibits a dose-response of approximately 90,000 dpm from 0 to 1000 ng/mL of D-amphetamine or D-methamphetamine with a minimum detectable dose for either drug of approximately 25 ng/mL. With a cut-off level of 500 ng/mL, the assay gave a positive result for 100% of the 111 clinical samples containing GC/MS confirmed (at or above the NIDA GC/MS cut-off values) levels of amphetamine and/or methamphetamine. Eighty-eight samples that screened negative in a clinical laboratory were all negative in the assay. Nineteen samples which were incorrectly identified as positive by other commercially available amphetamine assays were negative in this RIA.